1. Field of the Invention
The invention relates to a process for the production of membranes based on silicic acid heteropolycondensates, which makes possible the direct production even of very thin membrane layers on a support.
2. Description of the Prior Art
Membranes based on silicic acid heteropolycondensates exhibit an excellent resistance to acids and organic solvents and are also quite stable within the pH range up to about 10. In addition, their structural properties, for example hydrophilic or hydrophobic character, porosity and the like, can be varied within wide ranges if the nature and concentration of the starting compounds and catalysts and also the conditions of preparation are chosen suitably.
It is known from U.S. Pat. No. 4,238,590 to convert silicic acid heteropolycondensates into membranes by mechanically cutting the polycondensates, which are produced in compact blocks, to give very thin slices which are then employed as membranes without further treatment or after prior smoothing. However, since the silicic acid heteropolycondensates are often insufficiently elastic, the slices of membrane break when cut, and the membrane surface required is also not achieved in most cases by this method.
A further process for the production of membranes consists in pouring a solution of the silicic acid heteropolycondensate in an organic solvent onto a plane support and allowing the solvent to evaporate. However, the membrane film obtained in this manner is not reinforced and is therefore insufficiently mechanically stable.
U.S. Pat. No. 4,374,933 describes another process for the production of porous membranes based on silicic acid heteropolycondensates at the interface between an organic phase and an aqueous phase. However, since the resulting membranes contain a great deal of water by virtue of contact with an aqueous phase, there is a risk, when they are dried, of excessive shrinking and associated crack formation.
The hydrolytic polycondensation of the starting components to form silicic acid heteropolycondensates takes place with loss of substance (elimination of water and loss of solvent), so that shrinkage of the polycondensates inevitably results. Hitherto, therefore, it has not been considered possible to carry out the production and drying of the membrane directly on a supporting or reinforcing base: on the contrary, it had been expected that the membrane therein would tear under the arising stress.
It has now been found, surprisingly, that, when the starting components are reacted on the surface of a support, a membrane film is formed, which, on drying, remains coherent and does not tear in spite of the shrinkage to be expected. It has also been found that, particularly if one or more reactants are brought direct to the surface of the support via a gas phase, membranes having excellent physical and mechanical properties are obtained in the form of a thin surface film.